The present invention relates to a method for the analysis, in particular, of gaseous media by means of microwave absorption, with microwave radiation of at least one frequency being generated, and to an apparatus for the analysis of gaseous media based on the absorption of microwaves, particularly with the use of the Stark effect, primarily for implementing the method, the apparatus including at least one microwave transmitter, at least one measuring cell, at least one detector, amplifying and display devices for the measurement signal, a regulating device for the microwave transmitter and a Stark generator.
It is known to analyze gaseous media, possibly also after conversion of a solid substance or a liquid into the gaseous state, by absorption of microwaves within certain characteristic frequency ranges by the excitation of rotational transitions in molecules of the media to be examined.
In order to reduce the frequency line width, the examination takes place in the low pressure range. The absorption lines are split by means of the Stark effect, an alternating Stark voltage preferably being applied to increase detection sensitivity. Such a process is basically highly selective and does not require preparatory operations, such as ionization or chemical reactions, on the substances to be examined.
In the past, the examination method could practically be used only in a laboratory setting. Industrially usable apparatus was not available since the prior art apparatus was expensive because of its structural, mechanical and electrical configuration, often included sensitive components and it was a problem or involved high expenditures to stay within critical parameters.
For example, it is necessary to stabilize the frequency of the transmitted microwave, and thus to regulate in the actual sense its frequency to the absorption maximum of the molecule of interest, possibly under consideration of the Stark voltage that is acting on it. For this purpose, it has been proposed to feed part of the high frequency of the microwave radiation, supplied with the aid of a directional coupler, to a mixer diode, to there compare it with the harmonic of a stable low frequency transmitter as generated, for example, by a quartz crystal, to bind the microwave frequency to the harmonic of the lower frequency by synchronization with a PLL circuit and to thus stabilize the high frequency of the microwave on the absorption line of the component to be measured.
It has also been proposed to additionally modulate the frequency of the microwave transmitter, to transmit part of the microwave radiation through a reference cell containing the component to be measured, with the splitting of the microwave radiation again requiring the use of a directional coupler. The microwave transmitter is then fixed to the absorption line of the component to be measured with the aid of the derived signal at a reference cell detector required in addition to the detector of the measuring cell.
The drawback of such a transmitter modulation is the certainly occurring increase in noise at the detector. Particularly because of the requirement for such complicated frequency stabilization, prior art methods and apparatus were too expensive so that competing measuring methods and apparatus were more economical. As a whole, the prior art methods and apparatus were unsuitable particularly for process measuring techniques.